Well the delay is mostly because once you have gone full sex, there isn't much more to do other than mix and match the threesomes or throw in minor kinks that some people get upset about because they aren't into them.This seems likely to happen at Relationship level 4. The dev seems to enjoy teasing the players a bit, coming up with everything short of PIV intercourse with the family members.
Why 4 and not 5? Sort of the "ICSTOR's pussy" version of "Chekhov's gun". The teaser image suggests 4 way sex between MC, Linda, Caroline and Sara will happen at some point. My assumption is that you have to bring everyone's relationship level to 5(and probably develop the storyline with Judy) to bring that about. Otherwise the dev is kind of a shmuck to sell the game using an image/scene that's nowhere in the game.![]()
Longer time between first sexual act and full on sex the better the story usually is being built.